To assess the visual perception of the resolution (pixel density) of a rasterized (digital) image, it is important to know that the average human eye has a resolution of about one arc minute. But this is true only in the center and only when the viewer is rested and relaxed. Thus, the visible screen resolution is strongly dependent on the viewing distance, as the following table illustrates. The pixel density is usually measured in pixels per inch (PPI), i.e. as the number of pixels across a distance of 25.4 mm.

Viewing Distance[m]

Eye Resolution [mm]

Eye Resolution [PPI]

Typical Application

0,25

0,07

350

Handy, display of a photo camera

0,35

0,10

250

Reading distance (tablet computer, magazine)

0,5

0,15

175

Computer monitor

1,5

0,44

58

Small television set

3

0,87

29

Large television set

10

2,9

9

Cinema

Eye resolution means the eye's capability to differentiate two adjacent pixels from the given viewing distance.

Pixels or dots?

It has been known for nearly 200 years, that all colors visible to the human eye can be produced by superimposing the light of the three primary colors red, green and blue. Since October 1953 there is color TV in the U.S. (since 1967 also in Germany), which is based on this principle. Every visible picture element (each pixel) consists of three small sub-pixels that glow in the primary colors. Originally the pixels were colored phosphor layers of a picture tube, now they are elements of a liquid crystal display (LCD) or organic light emitting diodes (OLED).Another method to produce color images is used in some video projectors. Here a color wheel with different color fields rotates at high speed in front of the lamp and the image is quickly switched repeatedly between the three colors according to the position of the color wheel.

Ultimately, it does not matter whether the color results from a spatial distribution (TV, LCD monitors, plasma screens) or by temporal distribution (DLP projector) of the three primaries: For reasonable viewing distance, our eyes can not resolve individual sub pixels and experiences a homogeneous color image.

For the determination of the resolution visible to our eyes, only the color pixel resolution is relevant and not how the color was created. A control of the individual sub-pixels by computer is not even possible: the individual lighting elements are entirely controlled at the hardware level. Since all three primary colors together are needed for the representation of black & white images, only the resolution of the full pixel determines the sharpness impression on the viewer.

This is an universal principle followed since the invention of color television. For example, DVD resolution is 720 columns and 576 rows, which of course means colored dots (pixels). HDTV comes in two versions (1280 x 720 pixels and 1920 x 1080 pixels) and that again counts complete pixels only.The resolution of computer monitors is often 1920 x 1200 pixels, Apple's iPhone 5 has a retina display with 1136 x 640 pixels, etc. Always and everywhere complete, colored pixels are counted.

Only the resolution of monitors in photo and video cameras is stated in another way by the manufacturers.

Some years ago an apparently clever marketing guy came up with the idea that numerical values ​​in the catalog would look better if they were higher. So he simply multiplied the number of pixels with three and cheeky claimed this were the resolution achieved. After some time other manufacturers had to follow suit for competitive reasons, and so this bad habit has now naturalized in photo and video equipment.Even some photography magazines follow this doctrine (e.g. the German Chip FotoVideo) while others refuse and recalculate the absurd manufacturer values down to real values (divide by three), for example the German Colorfoto. To make a clear distinction between real pixels and this deceptive misrepresentation, they are referred to as sub-pixels or dots.

Left: Closeup of a Sony HDTV LCD television. Each pixel is made of nine sub-pixels (instead of three), namely three stacked elements per primary color (red, green, blue). This results in better control of the brightness levels. If you look even closer then you can see that each of the nine sub-pixels is again composed of four bright areas.Nevertheless, even the resolution of this TV is "just" 1920 x 1080 pixels. The 3x3x4 sub-pixels per pixel are controlled purely on hardware level and do not improve the perceived image resolution.

Below left: A single pixel in all of its glory.Below right: This is what pixels of an old CRT-TV look like.

Pixel peeping

The Video (640 x 480, 6,3 MB) shows a high resolution detail of the display of a modern 55" HDTV. It einen hochauflösenden Ausschnitt eines modernen 55" HDTV-Fernsehens. One can see how pixels and sub-pixels are composed and change with image content.

The film is encoded in a modern MPEG-4 format. Playback problems can usually be resolved by installing the latest Quick-Time Player from Apple (also available for Windows PCs).

Retina displays

The Apple company has launched the marketing term "retina display" for digital displays, when the resolution at typical viewing distance is better than the resolution of the human eye, which is about one minute of an arc. The following table shows typical conditions for popular devices. It is interesting to note that full HD TVs, which have been on the market for quite a while, easily meet the criterion of "retina displays". As a matter of fact, their relative resolution is much better than a current iPad Retina or iPhone 5. Against this background, the introduction of 4K TVs appears questionable.

Manu-facturer

Device

Horiz.[Pixel]

Vertical[Pixel]

Mega-Pixel

Diagonal[inch]

Resolution[PPI]

Viewing distance[m]

Requiredresolution[arc min]

RetinaDisplay?

Apple

iPhone 4S

960

640

0,614

3,5

330

0,25

1,06

Yes

Apple

iPhone 5

1136

640

0,727

4

326

0,25

1,07

Yes

Samsung

Galaxy SIII

1280

720

0,922 (*1)

4,8

306

0,3

0,95

Yes

Apple

iPad Mini

1024

768

0,786

7,9

162

0,35

1,54

No

Apple

iPad 2

1024

768

0,786

9,7

132

0,35

1,89

No

Apple

iPad Retina

2048

1536

3,149

9,7

264

0,35

0,95

Yes

Sony

Alpha 99

800 (*2)

512 (*2)

0,410

3

317

0,3

0,92

Yes

Canon

EOS 5D Mark III

720 (*2)

480 (*2)

0,346

3,2

270

0,3

1,08

Yes

Nikon

D800

740 (*2)

415 (*2)

0,307

3,2

265

0,3

1,10

Yes

Panasonic

GH3

640 (*2)

320 (*2)

0,205

3

239

0,3

1,22

No

Canon

XF100

740 (*2)

415 (*2)

0,307

3,5

242

0,4

0,90

Yes

Computer-Monitor

1024

768

0,786

20

64

0,5

2,37

No

Computer-Monitor

1920

1200

2,304

24

94

0,5

1,85

No

medium sized HDTV-television

1920

1080

2,074

42

52

1,6

1,04

Yes

Large HDTV-television

1920

1080

2,074

55

40

2

1,09

Yes

The viewing distance is the assumed, typical distance when using the respective device.The required resolution is measured when the device of a given pixel density is held at the viewing distance in order to discern individual pixels. If the required eye resolution is at or below the real eye resolution (about one minute of arc), then the device is called a Retina display.(* 1) The Samsung SIII uses only two of the three primary colors per pixel. Therefore, its color resolution is lower than indicated.(* 2) Estimated value based on specified total number of pixels and aspect ratio.